There are many instances in hydraulic control systems in which a lag network is connected between a hydraulic pressure line and a fluid control mechanism for restricting flow of fluid to the mechanism and thereby delaying and damping response of the control mechanism to fluctuations in fluid pressure at the fluid line. One example where a lag network of this character might be employed is in load-sensing pressure compensation of a variable displacement hydraulic pump of the type disclosed in U.S. Pat. No. 4,695,230. Delay and damping of the compensation control system helps eliminate pressure pulsations to the control mechanism, and thereby helps prevent oscillating movement of the pump displacement control under heavy pump load.
Conventionally, a lag network of the subject character comprises an orifice positioned in the hydraulic line to cooperate with a volume downstream of the orifice, formed by the line itself or by a separate accumulator, to restrict fluid flow. Such orifice/volume combination exhibits the desirable characteristic of attenuating the oscillating pressure on the volume resulting from oscillating flows passing through the orifice resistance. However, the orifice resistance to fluid flow is highly non-linear, and approaches zero as total fluid flow approaches zero. (The term "orifice resistance" refers to incremental resistance--i.e., a change in pressure divided by a change in flow about some steady operating flow through the orifice.) Thus, the filtering or attentuating property of the orifice/volume network fails at low flows because the orifice resistance approaches zero.
It is therefore a general object of the present invention to provide a hydraulic control system embodying a lag network between the primary fluid pressure line and the fluid control mechanism that exhibits or is characterized by a resistance to fluid flow that increases as fluid flow approaches zero. Another and related object of the invention is to provide a system of the described character that maintains resistance at low flow as described, while at the same time exhibiting either constant resistance or increasing resistance to flow as flow increases.
A further object of the invention is to provide a variable displacement pump control system that includes a pressure compensation network embodying such a lag network for controlling pump displacement as a function of load pressure.
Yet another object of the invention is to provide a bidirectional check valve for in-line connection in a hydraulic fluid system that achieves a more nearly constant resistance with changes in fluid flow, particularly at low flow.